Systems for recovering ammonia

ABSTRACT

Apparatus for removing ammonia from a gas stream includes primary and secondary scrubbers each of which includes a gas inlet, a gas outlet, and a slurry atomizer. The gas inlet of the primary scrubber is operatively connected to a gas stream having ammonia contained therein. A primary slurry feed system operatively connected to the slurry atomizer of the primary scrubber feeds to the slurry atomizer a primary slurry composition of a molybdic acid at a pH level. The gas inlet of the secondary scrubber is operatively connected to the gas outlet of the primary scrubber. A secondary slurry feed system operatively connected to a slurry atomizer of the secondary scrubber feeds to the slurry atomizer a secondary slurry composition of a molybdic acid at pH level. The pH level of the secondary slurry composition is lower than the pH level of the primary slurry composition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of co-pending U.S. patent applicationSer. No. 14/661,303, filed on Mar. 18, 2015, now allowed, which claimsthe benefit of U.S. provisional patent application Ser. No. 61/968,702,filed on Mar. 21, 2014, both of which are hereby incorporated herein byreference for all that they disclose.

TECHNICAL FIELD

This invention relates to gas scrubbing systems in general and morespecifically to systems for recovering ammonia contained in gas streams.

BACKGROUND

Gas scrubbing systems are well-known in the art and are used to remove awide range of compounds contained in untreated gaseous emissions fromvarious types of processes. In one particular application, gas scrubbingsystems have been developed to remove ammonia (NH₃) from gas streams,typically by contacting the gas streams with various types of packedbeds, spray chambers, and water jets. Because ammonia is alkaline, manytypes of ammonia scrubbing systems use an acid solution, typicallysulfuric acid, to remove or scrub the ammonia from the gas stream.

While such ammonia scrubbers are known and used in a wide variety ofapplications, they can be difficult to implement in certain applicationsdue to material handling problems, clogging, and filtering requirements.Consequently, improved systems and methods of removing ammonia from gasstreams are constantly being sought.

SUMMARY OF THE INVENTION

One embodiment of a method of removing ammonia from a gas stream mayinvolve the steps of: Providing a primary slurry composition of amolybdic acid at a first pH; providing a secondary slurry composition ofa molybdic acid at a second pH, the second pH being numerically lowerthan the first pH; atomizing the primary slurry composition in thepresence of the gas stream to produce a partially processed gas stream,quantities of ammonia in the gas stream combining with the molybdic acidin the primary slurry composition to form a further concentrated primaryammoniated slurry composition; and atomizing the secondary slurrycomposition in the presence of the partially processed gas stream toproduce a processed gas stream, additional quantities of ammonia in thepartially processed gas stream combining with the molybdic acid in thesecondary slurry composition to form a further concentrated secondaryammoniated slurry composition.

Another method of removing ammonia from a gas stream, may include thesteps of: Providing a primary slurry composition of a molybdic acid at apH level; providing a secondary slurry composition comprising molybdicacid at a pH level, the pH level of the secondary slurry compositionbeing numerically lower than the pH level of the primary slurrycomposition; atomizing the primary slurry composition in the presence ofthe gas stream to produce a partially processed gas stream, quantitiesof ammonia in the gas stream combining with the molybdic acid in theprimary slurry composition to form primarily ammonium dimolybdate; andatomizing the secondary slurry composition in the presence of thepartially processed gas stream to produce a processed gas stream,additional quantities of ammonia in the partially processed gas streamcombining with the molybdic acid in the secondary slurry composition toform primarily ammonium heptamolybdate.

Also disclosed is apparatus for removing ammonia from a gas stream thatincludes a primary scrubber having a gas inlet, a gas outlet, and aslurry atomizer. The gas inlet of the primary scrubber is operativelyconnected to the gas stream having ammonia contained therein. A primaryslurry feed system operatively connected to the slurry atomizer of theprimary scrubber feeds to the slurry atomizer a primary slurrycomposition of a molybdic acid at a pH level. A secondary scrubberincludes a gas inlet, a gas outlet, and a slurry atomizer. The gas inletof the secondary scrubber is operatively connected to the gas outlet ofthe primary scrubber. A secondary slurry feed system operativelyconnected to the slurry atomizer of the secondary scrubber feeds to theslurry atomizer a secondary slurry composition of a molybdic acid at pHlevel, the pH level of the secondary slurry composition being lower thanthe pH level of the primary slurry composition.

In operation, the apparatus atomizes the primary slurry composition inthe primary scrubber. The atomized primary slurry composition absorbsquantities of ammonia contained in the gas stream to produce a partiallyprocessed gas stream. The partially processed gas stream exits theprimary scrubber via the gas outlet and enters the secondary scrubbervia the gas inlet. The apparatus atomizes the secondary slurrycomposition in the secondary scrubber. The atomized secondary slurryabsorbs additional quantities of ammonia contained in the partiallyprocessed gas stream to produce a processed gas stream. The processedgas stream exits the secondary scrubber via the gas outlet.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative and presently preferred exemplary embodiments of theinvention are shown in the drawings in which:

FIG. 1 is a schematic representation of an ammonia recovery systemaccording to one embodiment of the invention;

FIG. 2 is a schematic representation of a second embodiment of anammonia recovery system; and

FIG. 3 is a schematic representation of a third embodiment of an ammoniarecovery system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of an ammonia recovery system 10 according to theteachings of the present invention is schematically illustrated in FIG.1 and may comprise at least two scrubbers, a first or primary scrubber12 and a secondary scrubber 14. Alternatively, three or more scrubberscould be used in other embodiments. The primary and secondary scrubbers12 and 14 may be operatively associated with respective primary andsecondary slurry feed systems 16 and 18. The first or primary slurryfeed system 16 may be operatively connected to a slurry atomizer 20provided within the primary scrubber 12. Similarly, the secondary slurryfeed system 18 may be operatively connected to a slurry atomizer 22provided within the secondary scrubber 14. The primary and secondaryslurry feed systems 16 and 18 feed respective primary and secondaryslurry compositions 24 and 26 to the respective slurry atomizers 20 and22. As will be discussed in much greater detail herein, the primary andsecondary slurry compositions 24 and 26 both may comprise molybdic acid,but at different pH and specific gravity levels. Generally speaking, thepH of the first or primary slurry composition 24 will be numericallygreater than the pH of the secondary slurry composition 26. Conversely,the specific gravity of the primary slurry composition 24 will be lowerthan the specific gravity of the secondary slurry composition 26.

The system 10 may be operated in accordance with a method 28 in order toremove ammonia (NH₃) contained in a gas stream 30. Briefly, the method28 may involve providing the first or primary slurry composition 24 tothe atomizer 20 provided in the primary scrubber 12. As alreadymentioned, the primary slurry composition 24 may comprise molybdic acidhaving a pH that is controlled to be generally greater than about 7,preferably in a range of about 7 to about 9.5, and more preferably in arange of about 8 to about 8.8. The specific gravity of the primaryslurry composition 24 is controlled to be about 1.3 or lower, preferablyin a range of about 1.2 to about 1.3.

The primary slurry composition 24 is atomized in the presence of the gasstream 30 which may be fed into the primary scrubber 12 via a gas inlet32. The atomized primary slurry composition 24 absorbs quantities ofammonia contained in the gas stream 30, forming a partially processedgas stream 34 as well as a further concentrated primary ammoniatedslurry composition 36. The partially processed gas stream 34 exits theprimary scrubber 12 via gas outlet 38, whereas the further concentratedprimary ammoniated slurry composition 36 may be returned to the primaryslurry feed system 16, as shown.

Significantly, the ammonia absorption process in the primary scrubber 12is terminated before completion, i.e., before all (or substantially all)of the ammonia has been removed from the gas stream 30. In oneembodiment of the invention, the absorption process is terminated whenabout 50-60% by volume of the ammonia has been removed from the gasstream 30. Also significantly, the absorption process occurring in theprimary scrubber 12 operates in an ADM absorption mode, wherein theammonia from the gas stream 30 combines with the molybdic acid in theprimary slurry composition 24 to form primarily ammonium dimolybdate,(NH₄)₂Mo₂O₇ or “ADM”.

The partially processed gas stream 34 from the first or primary scrubber12 is then directed to the secondary scrubber 14, e.g., via a gas inlet40 provided on secondary scrubber 14. The secondary slurry feed system18 provides the secondary slurry composition 26 to the slurry atomizer22 in the secondary scrubber 14. As was the case for the primary slurrycomposition 24, the secondary slurry composition 26 may comprisemolybdic acid. The pH of the secondary slurry composition 26 iscontrolled to be generally greater than about 6, and more preferably ina range of about 6-6.8. The specific gravity of the secondary slurrycomposition 26 is controlled to be about 1.4 or lower, preferably in arange of about 1.3 to about 1.4. The atomized secondary slurrycomposition 26 absorbs additional quantities of ammonia remaining in thepartially processed gas stream 34, forming a fully processed gas stream42 and a further concentrated secondary ammoniated slurry composition44. The fully processed gas stream 42 exits the secondary scrubber 14via gas outlet 46, whereupon it may be vented to the atmosphere. Thefurther concentrated secondary ammoniated slurry composition 44 may bereturned to the secondary slurry feed system 18.

Unlike the absorption process in the primary scrubber 12, the absorptionprocess in the secondary scrubber 14 operates in an AHM absorption mode,wherein the ammonia from the partially processed gas stream 34 combineswith the molybdic acid in the secondary slurry composition 26 to formprimarily ammonium heptamolybdate, (NH₄)₆Mo₇O₂₄ or “AHM”, as opposed toammonium dimolybdate. As will be described in much greater detailherein, the different absorption modes in the primary and secondaryscrubbers 12 and 14 is due to the different pH levels of the primary andsecondary slurry compositions 24 and 26 and provides significantoperational and efficiency advantages.

In one embodiment, the primary slurry feed system 16 may be providedwith a bleed outlet 48 to allow quantities of the primary slurrycomposition 24 to be withdrawn or bled from the primary slurry feedsystem 16. The withdrawn primary slurry composition 24 may be furthercombined with anhydrous ammonia 46 before being directed to an ammoniarecovery system 52 as a feed liquor 54. Ammonia recovery system 52removes the captured ammonia contained the feed liquor 54. The secondaryslurry feed system 18 may be provided with a make-up outlet 56 that isoperatively connected to the primary slurry feed system 16. The make-upoutlet 56 allows quantities of the secondary slurry composition 26 to beadded to the primary slurry feed system 16 to make up for the primaryslurry composition 24 withdrawn from the bleed outlet 48 of the primaryslurry feed system 16.

A significant advantage of the system and method of the presentinvention is that it provides a highly efficient means for recoveringammonia contained in gas streams without the material handling,clogging, and steam requirements of other types of ammonia recoverysystems. For example, by selectively operating the scrubbers 12 and 14in two different ammonia absorption modes, i.e., in the ADM absorptionmode in the primary scrubber 12 and the AHM absorption mode in thesecondary scrubber 14, the present invention avoids the formation ofexcessive precipitates in the primary and secondary slurry compositions,which can cause material handling, clogging, and filtering problems. Forexample, ammonium heptamolybdate, AHM, has a higher solubility in waterthan does ammonium dimolybdate. Consequently, the secondary slurrycomposition 26 is capable of absorbing the contained ammonia remainingin the partially processed gas stream 34 without the formation ofsignificant precipitates. While ADM is has a lower solubility in waterthan does AHM, the lower specific gravity of the primary slurrycomposition 24 ameliorates the problems that would otherwise beassociated with the build-up of excessive solids (i.e., precipitates) inthe primary slurry compositions.

Still yet another advantage associated with the use of the ADMabsorption mode in the primary scrubber 14 is that the preferentialformation of ADM over AHM means that the primary slurry composition 24is capable of absorbing, on an atomic basis, approximately double theamount of ammonia compared to what can be absorbed by the formation ofAHM. Stated another way, the ADM absorption mode is particularlywell-suited for use in the primary scrubber 12, wherein the amount ofcontained ammonia in the gas stream 30 is high. Similarly, the AHMabsorption mode is well-suited for use in the secondary scrubber 14 inorder to keep the vapor pressure of ammonia low and to avoid theformation of excessive solids in the secondary slurry composition 26.

Having briefly described various embodiments of the systems and methodsof ammonia recovery according to the present invention, as well as someof the more significant attributes and advantages thereof, variouspreferred embodiments of the systems and methods of recovering ammoniawill now be described in detail. However, before proceeding with thedescription it should be noted that neither the following descriptionnor the claimed process should be regarded as restricted to anyparticular operational parameters, including reagent quantities, theorder of reagent addition, reaction conditions, and other numericalvalues, unless otherwise indicated. Still further, it should be notedthat specific reaction parameters and other operational factors may beoptimized in a given situation, taking into account environmentalfactors, production-scale requirements, and the like, using routinepreliminary pilot testing. The discussion provided herein involves oneor more preferred embodiments which are designed to provide optimumresults and shall not be considered limiting or restrictive.

With reference back now to FIG. 1, one embodiment of the ammoniarecovery system 10 is shown and described herein as it could be used torecover ammonia contained in a gas stream 30 produced by a chemicalprocessing plant. In the particular embodiment shown in FIG. 1, the gasstream 30 from the chemical processing plant is a combination of acalciner vent stream 58 and a tank vent stream 60. The calciner ventstream 58 is produced by a calcination process and typically comprises amixture of air, water vapor, ammonia, and small amounts of carbondioxide. The calciner vent stream 58 is provided at temperatures rangingfrom about 140° C. to about 150° C. (about 285-300° F.). The tank ventstream 60 is produced by a mixing tanks used in production and usuallycomprises a mixture of air, water vapor, and ammonia. The tank ventstream 60 is provided at temperatures ranging from about 27° C. to about32° C. (about 80-90° F.). The resulting gas stream 30, i.e., resultingfrom combination of the calciner vent stream 58 and the tank vent stream60, will generally comprise a mixture of air, water vapor, ammonia, andcarbon dioxide at a temperature of about 96° C. (about 205° F.). Theamount of ammonia in the gas stream 30 may range from about 0.5% byvolume to about 5% by volume, with ammonia concentrations ranging fromabout 2% to about 3% by volume being typical. After being collected andcombined, the gas stream 30 is fed into the gas inlet 32 of the primaryscrubber 12.

As mentioned above, certain embodiments of the present invention mayutilize two scrubbers, e.g., primary scrubber 12 and secondary scrubber14 connected in a series arrangement. However, other embodiments may useadditional scrubbers (i.e., or more), either in series or in parallelarrangements, depending on a wide range of factors, as would becomeapparent to persons having ordinary skill in the art after having becomefamiliar with the teachings provided herein. Consequently, the presentinvention should not be regarded as limited to embodiments using onlytwo scrubbers.

The primary scrubber 12 may comprise a generally vertically oriented,elongated chamber having a slurry atomizer 20, a gas inlet 32, and a gasoutlet 38, arranged substantially as shown in FIG. 1. The primaryscrubber 12 may also be provided with a collection sump 62 suitable forcollecting the further concentrated primary ammoniated slurry 36. Ifdesired, the collection sump 62 may be provided with an agitator (notshown) to prevent settling and ensure a thorough dispersal of the solidsin the further concentrated primary ammoniated slurry composition 36.

The secondary scrubber 14 may be similar to the primary scrubber 12 andmay also comprise a generally vertically oriented, elongated chamberhaving a slurry atomizer 22, a gas inlet 40, and a gas outlet 46. Thegas inlet 40 of secondary scrubber 14 may be connected to the gas outlet38 of primary scrubber 12 so that the partially processed gas stream 34from the primary scrubber 12 may be fed into the secondary scrubber 14.The gas outlet 46 from the secondary scrubber 14 may be vented to theatmosphere via a suitable blower assembly 64 and vent stack 66. Thesecondary scrubber 14 may also be provided with a collection sump 68 forcollecting the further concentrated secondary ammoniated slurry 44. Thecollection sump 68 also may be provided with an agitator (not shown) toprevent settling and ensure a thorough dispersal of the solids in thefurther concentrated secondary ammoniated slurry composition 44.

The primary and secondary scrubbers 12 and 14 may be operativelyconnected to respective primary and secondary slurry feed systems 16 and18. As briefly described above, the primary slurry feed system 16provides a primary slurry composition 24 to the slurry atomizer 20 ofthe primary scrubber 12. Similarly, the secondary slurry feed systemprovides a secondary slurry composition 26 to the slurry atomizer 22 ofthe secondary scrubber 14. In addition, the primary and secondary slurryfeed systems 16 and 18 may be configured to receive the furtherconcentrated primary and secondary ammoniated slurry compositions 36 and44 from the respective primary and secondary scrubbers 12 and 14, asbest seen in FIG. 1.

In one embodiment, the primary slurry feed system 16 may comprise aprimary tank 70 sized to receive a quantity of the primary slurrycomposition 24. The primary tank 70 may be provided with one or moreagitator systems 72 to ensure a thorough dispersal of the solids in theprimary slurry composition 24. A pump system 74 connected between theprimary tank 70 and the slurry atomizer 20 in the primary scrubber 12may be used to deliver the primary slurry composition 24 to the slurryatomizer 20. The primary tank 70 may also be provided with a levelcontrol system 76 which, in combination with valve 78, maintains theprimary slurry composition 24 in primary tank 70 at the desired level.

As mentioned earlier, the primary slurry feed system may also beprovided with bleed outlet 48 which allows quantities of the primaryslurry composition 24 to be withdrawn from the primary slurry feedsystem 16. In one embodiment, the withdrawn slurry composition 24 may bedirected to a surge tank for subsequent handling and ammonia removal, aswill be described in greater detail below.

The primary slurry feed system 16 may also be provided with a pH controlsystem 82 for controlling the pH of the primary slurry composition 24.The pH control system 82 may comprise a pH sensor 84 provided in theline connecting pump 74 and slurry atomizer 20. PH sensor 84 senses thepH of the primary slurry composition 24 being fed to the atomizer 20. Avalve 86 may be positioned in the bleed outlet 48 and opened and closedas necessary to bleed off or remove quantities of the primary slurrycomposition 24 to control the pH of the primary slurry composition 24 ina manner that will be described in further detail below.

The primary slurry feed system 16 may also comprise an optional surgetank 88. The surge tank 88 may be connected to the primary tank 70 via apump 90 and may be used to supply make-up quantities of the primaryslurry composition 24 to the primary tank 70. In one embodiment, surgetank 88 may be configured to receive a supply of primary slurrycomposition 24 from a primary slurry source 92. Surge tank 88 also maybe configured to receive quantities of the secondary slurry composition26 from the make-up outlet 56 from secondary slurry feed system 18, asdepicted in FIG. 1. Surge tank 88 also may be provided with an agitatorsystem 94 to ensure a thorough dispersal of the solids in the primaryslurry composition 24.

The secondary slurry feed system 18 may be similar to the primary slurryfeed system 16 and may comprise a secondary tank 96 sized to receive aquantity of the secondary slurry composition 26. The secondary tank 96may be provided with an agitator 98 to ensure a thorough dispersal ofthe solids in the secondary slurry composition 26. A pump 11 may beconnected between the secondary tank 96 and the slurry atomizer 22 inthe secondary scrubber 14. Pump 11 delivers the secondary slurrycomposition 26 to the secondary scrubber 14 whereupon it is atomized bythe slurry atomizer 22. The secondary tank 96 may also be provided witha level control system 13 which, in combination with valve 15 may beused to maintain the secondary slurry composition 26 in secondary tank96 at the desired level.

The secondary slurry feed system 18 may be provided with make-up outlet56 which allows quantities of the secondary slurry composition 26 to bewithdrawn from the secondary slurry feed system 18 and fed to theprimary slurry feed system 16. In the particular embodiment shown anddescribed herein, the withdrawn slurry composition 26 may be directed tosurge tank 88.

The secondary slurry feed system 18 may also be provided with a pHcontrol system 17 for controlling the pH of the secondary slurrycomposition 26. The pH control system 17 may comprise a pH sensor 19provided in the line connecting pump 11 and slurry atomizer 22. Sensor19 senses the pH of the secondary slurry composition 26 being fed to theatomizer 22 of the secondary scrubber 14. A valve 21 positioned in themake-up outlet 54 may be opened and closed as necessary to bleed off orremove quantities of the secondary slurry composition 26 to control thepH of the secondary slurry composition 26.

The secondary slurry feed system 18 may also comprise an optional surgetank 23. The surge tank 23 may be connected to the secondary tank 96 viaa pump 25 and may be used to supply make-up quantities of the secondaryslurry composition 26 to the secondary tank 96. In one embodiment, surgetank 23 may be configured to receive a supply of secondary slurrycomposition 26 from a secondary slurry source 27. Surge tank 23 also maybe configured to receive quantities of water (e.g., deionized water) 29,which may be used to control the specific gravity and/or pH of therespective slurries 24 and 26. Surge tank 23 may be provided with anagitator 31 to ensure a thorough dispersal of the solids in thesecondary slurry composition 26.

As mentioned above, quantities of the primary slurry composition 24 thatare bled from the primary slurry feed system may be directed to a surgetank 80 for holding and/or additional processing before being sent to anammonia recovery system 52 via pump 33. By way of example, in oneembodiment, the ammonia recovery system 52 may comprise one or morefilters and/or particle separation systems for removing (by filtering)any impurities that may be precipitated when the molybdic acid (MoO₃) isinitially dissolved. ADM is crystallized from the ammonium molybdatesolution and some of that solution left over is used along with excesswater and ammonia collected during crystallization to dissolveadditional amounts of MoO₃ for subsequent use in the scrubbers. Surgetank 80 may be provided with an agitator 35 and may also be connected tothe supply of anhydrous ammonia 46. Anhydrous ammonia 46 may be added asnecessary to form additional ammonium molybdates within the slurry 24,thereby enhancing the subsequent ammonia recovery process.

The system 10 may be operated in accordance with method 28 to removeammonia contained in gas stream 30. As an initial matter, the primaryand second slurry compositions 24 and 26 are provided to the respectiveprimary and secondary slurry feed systems 16 and 18. In the particularembodiment shown and described herein, the primary slurry composition 24may be provided by primary slurry source 92 from the chemical plant. Theprimary slurry composition 24 from the slurry source 92 is formed bymixing molybdenum trioxide (MoO₃) with water. The resulting molybdicacid will be quite acidic, having a pH in a range of about 2.2-2.3. Tothis mixture, ammonia (NH₃) may be added to increase the pH to thedesired level. Sufficient water also may be added to achieve the desiredspecific gravity. Upon initial start-up, i.e., before the primary slurryfeed system 16 contains the primary slurry composition 24, the primaryslurry source 92 will be operated to provide the primary slurrycomposition 24 with the desired pH and specific gravity levels.Thereafter, the primary slurry source 92 may be operated to provide theprimary slurry composition 24 with slightly different pH and specificgravity levels as the primary slurry 24 in the primary slurry feedsystem 16 will be intermixed with further concentrated primaryammoniated slurry 36 from the primary scrubber 12, as well as make-upquantities of the secondary slurry 26, as described.

By way of example, in the particular embodiment shown in FIG. 1, uponinitial start-up, a primary slurry source 92 will be operated to providethe primary slurry 24 with a pH greater than about 7, preferably in arange of about 7 to about 9.5, and more preferably in a range of about 8to about 8.8. Similarly, the specific gravity of the primary slurrycomposition 24 from the primary slurry source 92 will have a specificgravity of about 1.3 or lower, preferably in a range of about 1.2 toabout 1.3.

Thereafter, once the system 10 has been operated for a period of timesufficient to reach equilibrium, the primary slurry source 92 may beoperated to provide additional quantities of slurry at slightlydifferent pH and specific gravity levels. By way of example, in oneembodiment, the primary slurry source 92 may be operated to provide theslurry at a pH of about 6 and a specific gravity of about 1.4. Again,when mixed with the other slurry streams, i.e., the further concentratedprimary ammoniated slurry 36 and make-up quantities of secondary slurry26, the primary slurry composition 24 provided to the primary scrubber12 will have pH and specific gravity levels within the ranges specifiedherein for steady state operation.

The initial quantities of the secondary slurry composition 26 may beprovided in a similar manner. For example, in the particular embodimentshown in FIG. 1, the secondary slurry composition 26 may be provided bya secondary slurry source 27 also from the chemical plant. The secondaryslurry composition 26 from the secondary slurry source 27 also may beformed by mixing molybdenum trioxide (MoO₃) with water, with ammoniaadded to increase the pH to the desired level. Sufficient water, e.g.,from deionized water source 29 may also be added to achieve the desiredspecific gravity. However, and unlike the case for the primary slurrycomposition 24 from the primary slurry source 92, the secondary slurrysource 27 may be operated to provide the secondary slurry composition 26at the specific gravity and pH levels required during steady stateoperation. That is, it will typically not be necessary to provide theinitial slurry amounts at different pH and/or specific gravity levels.Thus, in one embodiment, the secondary slurry composition 26 provided bythe secondary slurry source 27 will have a pH of at least about 6 andpreferably in a range of about 6 to about 6.8. The specific gravity ofthe secondary slurry 26 should be below about 1.4, and preferably in arange of about 1.3 to about 1.4. By way of example, during operation thesecondary slurry source 27 will provide slurry having a pH of about 6and a specific gravity of about 1.3.

Before proceeding with the description, it should be noted that thepermissible specific gravity levels for the primary slurry composition24 are related to temperature. Higher slurry temperatures will allowhigher specific gravity levels to be used without problems due toexcessive precipitation in the slurry composition. Conversely, lowerslurry temperatures will require lower specific gravity levels in orderto avoid problems due to excessive precipitation in the slurry. Thespecific gravity levels provided herein are suitable for primary slurrycompositions 24 having temperatures in the range of about 54° C. toabout 66° C. (about 130-150° F.)

Once sufficient quantities of the primary and secondary slurrycompositions 24 and 26 have been provided, the system 10 is ready tobegin removing ammonia from the gas stream 30. As mentioned above, inthe particular embodiment shown and described herein the gas stream 30is a combination of the calciner vent stream 58 and the tank vent stream60. The calciner vent stream 58 is produced by a calcination process andcomprises a mixture of air, water vapor, ammonia, and small amounts ofcarbon dioxide. The calciner vent stream 58 typically will be at atemperature ranging from about 140° C. to about 150° C. (about 285-300°F.). The tank vent stream 60 is produced by a mixing tanks used inproduction and comprises a mixture of air, water vapor, and ammonia. Thetank vent stream 60 usually will be at a temperature ranging from about27° C. to about 32° C. (about 80-90° F.).

The resulting gas stream 30, i.e., resulting from combination of thecalciner vent stream 58 and the tank vent stream 60, will generallycomprise a mixture of air, water vapor, ammonia, and carbon dioxide at atemperature of about 96° C. (about 205° F.). The amount of ammonia inthe gas stream 30 may range from about 0.5% by volume to about 5% byvolume, with ammonia concentrations ranging from about 2% to about 3% byvolume being typical. After being collected and combined, the gas stream30 is fed into the gas inlet 32 of the primary scrubber 12.

Once in the primary scrubber 12, the gas stream 30 will contact theprimary slurry composition 24 being discharged by the slurry atomizer20. In one embodiment, the pump 74 will provide the primary slurry 24 ata flow rate and pressure sufficient to cause the slurry 24 to bedischarged at a differential pressure of about 0.69 bar (about 10 poundsper square inch, differential (psid)), which is sufficient to providegood atomization of the primary slurry 24 with the particular slurryatomizer 20. As the atomized primary slurry 24 contacts the gas stream30, ammonia in the gas stream 30 will combine with the molybdic acid inthe primary slurry 24. As already mentioned, at the particular elevatedpH levels of the primary slurry composition (i.e., at a pH of at leastabout 7), the ammonia will combine with the molybdic acid to formprimarily ammonia dimolybdate, (NH₄)₂Mo₂O₇ or ADM, although otherammonium molybdates may also be formed, but to a much lesser degree. Theresulting further concentrated primary ammoniated slurry 36 will collectin the sump 62 of primary scrubber 12, whereupon it will be returned toprimary tank 70 of the primary slurry feed system 16.

The ammonia absorption process in the primary scrubber 12 is terminatedbefore completion, i.e., before all (or even before substantially all)of the ammonia has been removed from the gas stream 30. The absorptionprocess may be terminated when about 50% by volume to about 60% byvolume of the ammonia has been removed from the gas stream 30. Thus, inan embodiment wherein the gas stream 30 contains about 2% by volumeammonia, the resulting partially processed gas stream 34 will containabout 0.2% by volume ammonia.

The partially processed gas stream 34 from the primary scrubber 12 isnext directed to the secondary scrubber 14. The secondary slurry feedsystem 18 provides the secondary slurry composition 26 to the slurryatomizer 22 in the secondary scrubber 14. In one embodiment, pump 11provides the secondary slurry 26 at a flow rate and pressure sufficientto cause the slurry 26 to be discharged from the atomizer 22 at adifferential pressure of about 0.69 bar (about 10 pounds per squareinch, differential (psid)), which is sufficient to provide goodatomization of the secondary slurry composition 26 with the particularslurry atomizer 22 involved.

Like the primary slurry composition 24, the secondary slurry composition26 comprises molybdic acid. However, the pH of the secondary slurrycomposition 26 is controlled to be generally greater than about 6, andmore preferably in a range of about 6-6.8. The specific gravity of thesecondary slurry composition 26 is controlled to be about 1.4 or lower,preferably in a range of about 1.3 to about 1.4 at secondary slurrytemperatures ranging from about 54° C. to about 66° C. (about 130-150°F.). Higher secondary slurry temperatures will allow higher specificgravity levels to be used, whereas lower secondary slurry temperatureswill generally require lower specific gravity levels in order to avoidproblems due to excessive precipitation in the secondary slurrycomposition 26.

The atomized secondary slurry composition 26 absorbs additionalquantities of ammonia remaining in the partially processed gas stream34, forming a fully processed gas stream 42 and a further concentratedsecondary ammoniated slurry composition 44. The fully processed gasstream 42 exits the secondary scrubber 14 via gas outlet 46, whereuponit may be vented to the atmosphere via blower 60 and vent stack 66. Thefurther concentrated secondary ammoniated slurry composition 44 may bereturned to the secondary slurry feed system 18.

The absorption process in the secondary scrubber 14 operates in an AHMabsorption mode, wherein the ammonia from the partially processed gasstream 34 combines with the molybdic acid in the secondary slurrycomposition 26 to form primarily ammonium heptamolybdate, (NH₄)₆Mo₇O₂₄or “AHM”, as opposed to ammonium dimolybdate. Other types of ammoniatedmolybdates may also be formed, but to a much lesser degree than AHM.

In one embodiment, the secondary scrubber 14 removes about 75% by volumeto about 80% by volume of the ammonia contained in the partiallyprocessed gas stream 34. Accordingly, in an embodiment wherein the gasstream 30 contains about 2% by volume ammonia the partially processedgas stream 34 will contain about 0.2% by volume ammonia, whereas thefully processed gas stream 42 will contain only about 300 parts permillion by volume (ppmv) ammonia.

As briefly described above, the primary and secondary slurry feedsystems 16 and 18 are interconnected, with the secondary slurry feedsystem 18 providing slurry (e.g., via make-up outlet 56) to the primaryslurry feed system 16 to make up for primary slurry withdrawn from thebleed outlet 48. This arrangement provides for enhanced control of pHand specific gravity, and also reduces the slurry delivery requirementsfor the primary and secondary slurry feed sources 92 and 27.

More specifically, as the ammonia from the gas stream 30 is absorbed inthe primary scrubber 12, the pH and specific gravity of the primaryslurry composition 24 gradually increases, i.e., due to the return ofthe further concentrated primary ammoniated slurry 36 from primaryscrubber 12. As the pH increases, the pH control system 82 will openvalve 86 to withdraw or bleed off some of the primary slurry composition24, whereupon it will be fed to surge tank 80 for subsequent recovery ofthe ammonia contained therein. The slurry required to make up or replacethe withdrawn primary slurry 24 may come from the secondary slurry feedsystem 18 via surge tank 88. Additional quantities of make-up slurry mayalso come from the primary slurry source 92 as well. In any event, sincethe pH of the replacement slurry compositions will be lower than the pHof the primary slurry composition 24, the addition of the replacementslurry compositions will help maintain the pH of the primary slurrycomposition 24 at the desired level.

As mentioned earlier, the primary slurry feed system 16 may also beprovided with a level control system 76. Level control system 76 may beused to control the slurry level in tank 70 by opening valve 78 to allowadditional amounts of slurry 24 from surge tank 88 to be pumped into theprimary tank 70. In this regard, the level control system 76,functioning together with the pH control system 82 operate as a specificgravity control system and may be operated to also control the specificgravity of the primary slurry composition 24. More specifically, a highpH or tank level condition will cause valve 86 to open, thereby droppingthe level of the slurry 24 in tank 70. A high specific gravity willtrigger the addition of makeup water or aqua ammonia to the system(e.g., via water source 29 and/or slurry sources 92 and 27). A lowslurry level in tank 70 will trigger the addition of additional slurryfrom the surge tank 88.

The secondary slurry feed system 18 is similar to the primary feedsystem 16, with the exception that additional slurry and/or water isprovided by the slurry and water sources 27 and via surge tank 23. Asthe ammonia from the partially processed gas stream 34 is absorbed inthe secondary scrubber 14, the pH and specific gravity of the secondaryslurry composition 26 will gradually increase due to the return of thefurther concentrated secondary ammoniated slurry 44 from secondaryscrubber 14. As the pH increases, the pH control system 17 will openvalve 21 to withdraw some of the secondary slurry composition 26 andfeed it via surge tank 88 to the primary slurry feed system 16. Theslurry required to make up or replace the withdrawn secondary slurry 26may come from the secondary slurry source 27 via surge tank 23. Sincethe pH of the replacement slurry will be lower than the pH of the (nowfurther concentrated) secondary slurry composition 26, the addition ofthe replacement slurry will help maintain the pH of the secondary slurrycomposition 26 at the desired level.

As was the case for the primary slurry feed system 16, the secondaryslurry feed system 18 may be provided with a level control system 13.Level control system 13 may be used to control the slurry level in tank96 by opening valve 15 to allow additional amounts of slurry 26 fromsurge tank 23 to be pumped into the secondary tank 96. The level controlsystem 13, functioning together with the pH control system 17 may beoperated as a specific gravity control system to also control thespecific gravity of the secondary slurry composition 26. Morespecifically, a high pH or tank level condition will cause valve 21 toopen, thereby dropping the level of the slurry 26 in tank 96. A highspecific gravity will trigger the addition of makeup aqua ammonia orwater to the system (e.g., via water source 29). A low slurry level intank 96 will trigger the addition of additional slurry from the surgetank 23.

The ammonia captured by the primary and secondary slurry compositions 24and 26 may be recovered by an ammonia recovery system 52. In oneembodiment, ammonia recovery system 52 may comprise a Hoesch-typefiltering system which removes any impurities from the withdrawn slurry.More specifically, quantities of the primary slurry composition 24 thatare bled from the primary slurry feed system 16 may be directed to surgetank 80 for holding and/or additional processing before being sent toammonia recovery system 52 via pump 33. Surge tank 80 may be providedwith an agitator 35 and may also be connected to the supply of anhydrousammonia 46. Anhydrous ammonia 46 may be sparged into surge tank 80 inorder to increase the ammonia concentration in the slurry to thatrequired by the subsequent filter recovery process.

As mentioned above, other arrangements and configurations of the ammoniarecovery system are possible. For example, a second embodiment 110 of anammonia recovery system may involve the integration or combination ofvarious components in order to simplify the overall system andstreamline operation.

With reference now primarily to FIG. 2, the second embodiment 110 maycomprise a first or primary scrubber 112 and a secondary scrubber 114that are operatively associated with respective primary and secondaryslurry feed systems 116 and 118. The primary slurry feed system 116 maybe operatively connected to a slurry atomizer 120 provided within theprimary scrubber 112, whereas the secondary slurry feed system 118 maybe operatively connected to a slurry atomizer 122 provided withinsecondary scrubber 114. The primary and secondary slurry feed systems116 and 118 feed respective primary and secondary slurry compositions124 and 126 to the respective atomizers 120 and 122. The primary andsecondary slurry compositions 124 and 126 may comprise molybdic acid,but at different pH and specific gravity levels, as already describedfor the first embodiment 10.

The second embodiment 110 also may be used to remove ammonia containedin a gas stream 130 produced by a chemical processing plant. Gas stream130 may result from the combination of a calciner vent stream and a tankvent stream, neither of which is specifically illustrated in FIG. 2, butwhich may involve the same arrangement as that illustrated in FIG. 1.That is, gas stream 130 of the second embodiment 110 will generallycomprise a mixture of air, water vapor, ammonia, and carbon dioxide at atemperature of about 96° C. (about 205° F.). The amount of ammonia inthe gas stream 130 may range from about 0.5% by volume to about 5% byvolume, with ammonia concentrations ranging from about 2% to about 3% byvolume being typical. After being collected and combined, the gas stream130 is fed into the gas inlet 132 of primary scrubber 112.

The primary scrubber 112 may comprise a generally vertically oriented,elongated chamber having gas inlet 132 and a gas outlet 138. In thesecond embodiment 110, the gas inlet 132 may comprise a generallyelongated, tube-like structure in which the atomizer 120 is positionedat about a distal end 121 of inlet 132, as best seen in FIG. 2. Thisarrangement provides for enhanced and more efficient mixing of the inletgas stream 130 and primary slurry composition 124. In the FIG. 2embodiment, the collection sump and primary tank (e.g., 62 and 70) ofthe first embodiment 10 are combined or integrated to form a primaryslurry tank 170 defined by the bottom portion of primary scrubber 112.Primary slurry tank 170 contains the primary slurry composition 124 andalso receives and collects further concentrated primary ammoniatedslurry 136 formed in primary scrubber 112. In the second embodiment 110,primary slurry tank 170 may also be provided with an agitator 172 toprevent settling and ensure a thorough dispersal of the solids in theprimary slurry composition 124. Primary scrubber 112 may also beprovided with a demister 141 to remove quantities of liquid or mistcontained in the partially processed gas stream 134 before it exitsprimary scrubber 112, i.e., via gas outlet 138. Primary scrubber 112 mayalso be provided with a cleaning system 143 which may be operated fromtime-to-time to spray water 129 on demister 141 to prevent cloggingthereof.

Secondary scrubber 114 may be similar to the primary scrubber 112 andmay also comprise a generally vertically oriented, elongated chamberhaving a gas inlet 140 and a gas outlet 146. The gas inlet 140 ofsecondary scrubber 114 may be connected to the gas outlet 138 of primaryscrubber 112 so that the partially processed gas stream 134 from theprimary scrubber 112 may be fed into the secondary scrubber 114. As wasthe case for primary scrubber 112, gas inlet 140 of secondary scrubber114 may comprise a generally elongated, tube-like structure in which theatomizer 122 is positioned at about a distal end 151 of inlet 140. Thisarrangement provides for enhanced and more efficient mixing of thepartially processed gas stream 134 and secondary slurry composition 126.The gas outlet 146 from the secondary scrubber 114 may be vented to theatmosphere via a suitable blower assembly 164 and vent stack 166.

The secondary scrubber 114 may also comprise a combined or integratedarrangement in which the sump and secondary slurry tank (e.g., 68 and96) of the first embodiment 10 are combined to form a single secondarytank 196 defined by the bottom portion of secondary scrubber 114. Thecombined secondary tank 196 holds the secondary slurry composition 126and also collects further concentrated secondary ammoniated slurry 144formed in secondary scrubber 114. Secondary tank 196 may be providedwith an agitator 198 to prevent settling and ensure a thorough dispersalof the solids in the secondary slurry composition 126. Secondaryscrubber 114 may also be provided with one or more demisters 145 toremove quantities of liquid or mist contained in the fully processed gasstream 142 before it exits secondary scrubber 114 via gas outlet 146.Secondary scrubber 114 may also be provided with a cleaning system 147which may be operated from time-to-time to spray water 129 on demisters145 to prevent clogging thereof.

As briefly described above, the primary and secondary scrubbers 112 and114 may be operatively connected to respective primary and secondaryslurry feed systems 116 and 118. Primary slurry feed system 116 providesthe primary slurry composition 124 to the slurry atomizer 120 of theprimary scrubber 112. Similarly, the secondary slurry feed system 118provides the secondary slurry composition 126 to the slurry atomizer 122of the secondary scrubber 114.

More specifically, primary slurry feed system 116 may comprise a pumpsystem 174 connected between the primary tank 170 and the slurryatomizer 120 in the primary scrubber 112. Pump 174 delivers the primaryslurry composition 124 to the slurry atomizer 120 under sufficientpressure to provide good atomization. Primary tank 170 may also beprovided with a level control system 176 which, in combination withvalve 178, may be used to maintain the primary slurry composition 124 inprimary tank 170 at the desired level.

The primary slurry feed system 116 may be connected to a reagent tank123 via a pump 125. Reagent tank 123 supplies quantities of a make-upslurry composition 199 to primary and secondary tanks 170 and 196. Inone embodiment, reagent tank 123 may be configured to receive a supplyof make-up slurry composition 199 from a slurry source 127. Reagent tank123 also may be configured to receive water (e.g., deionized water) 129,which may be used to control the specific gravity and/or pH of themake-up slurry composition 199. Reagent tank 123 also may be providedwith an agitator system 131 to ensure a thorough dispersal of the solidsin the make-up slurry composition 199.

Primary slurry feed system 116 may also comprise a bleed outlet 148which allows quantities of the primary slurry composition 124 to bewithdrawn from the primary slurry feed system 116. The withdrawn slurrycomposition 124 may be directed to an ammonia recovery system 152 forsubsequent handling and ammonia removal, as will be described in greaterdetail below.

The primary slurry feed system 116 may also be provided with a pHcontrol system 182 for controlling the pH of the primary slurrycomposition 124. The pH control system 182 may comprise a pH sensor 184provided in the line connecting pump 174 and slurry atomizer 120. PHsensor 184 senses the pH of the primary slurry composition 124 being fedto atomizer 120. A valve 185 positioned in feed line from reagent tank123 may be opened and closed as necessary to add additional quantitiesof make-up slurry composition 199 to control the pH of the primaryslurry composition 124. Additional quantities of make-up slurrycomposition 199 will reduce or lower the pH of the slurry composition124.

Primary slurry feed system 116 may also be provided with a densitycontrol system 137 to control the density of the primary slurrycomposition 124 contained within primary tank 170. Density controlsystem 137 may comprise a density meter, such as a Coriolis densitymeter (not shown), and may be operatively connected to valves 183 and186. The density is lowered by opening valve 186, thereby increasing theflow rate of primary slurry composition 124 to the ammonia recoverysystem 152. Density control system 137 may also operate valve 183 tocontrol the flow of aqueous ammonia 181 into primary tank 170, therebyproviding further control over the density of the primary slurrycomposition 124.

The secondary slurry feed system 118 may be similar to the primaryslurry feed system 116. Secondary slurry feed system 118 may comprise apump 111 connected between the secondary tank 196 and the slurryatomizer 122 in the secondary scrubber 114. Pump 111 delivers thesecondary slurry composition 126 to the secondary scrubber 114 whereuponit is atomized by the slurry atomizer 122. The secondary slurry feedsystem 118 may be provided with make-up outlet 156 which allowsquantities of the secondary slurry composition 126 to be withdrawn fromthe secondary slurry feed system 118 and fed to the primary slurry feedsystem 116 via primary tank 170. Secondary tank 196 may be provided witha level control system 113 which, in combination with valve 115, may beused to maintain the secondary slurry composition 126 in secondary tank196 at the desired level.

The secondary slurry feed system 118 may also be provided with a pHcontrol system 117 for controlling the pH of the secondary slurrycomposition 126. The pH control system 117 may comprise a pH sensor 119provided in the line connecting pump 111 and slurry atomizer 122. Sensor119 senses the pH of the secondary slurry composition 126 being fed tothe atomizer 122 of the secondary scrubber 114. A valve 121 connected topump 125 of reagent tank 123 may be opened and closed as necessary toadd additional quantities of the make-up slurry composition 199 tocontrol the pH of the secondary slurry composition 126.

The density of the secondary slurry composition 126 may be monitored bya density monitoring system 149. Density monitoring system 149 maycomprise a density meter, such as a Coriolis density meter (not shown),and may be used to obtain an indication of the density of the secondaryslurry composition 126 contained in secondary tank 196. In the secondembodiment 110, the density or specific gravity of the secondary slurrycomposition may be indirectly controlled via the level control system113, i.e., by adding additional quantities of water 129 to secondarytank 196.

As mentioned above, quantities of the primary slurry composition 124that are bled from the primary slurry feed system 116 may be directed toan ammonia recovery system 152. As was the case for the first embodiment10, the ammonia recovery system 152 of the second embodiment 110 maycomprise one or more filters and/or particle separation systems forremoving (by filtering) any impurities that may be contained in feedliquor 154.

Finally, second embodiment 110 may also be connected to a supply ofaqueous ammonia 181. More specifically, the supply of aqueous ammonia181 may be connected to tank 170 via valve assembly 183. Aqueous ammonia181 may be used to provide further control of the specific gravityand/or pH of the primary slurry composition 124.

A third embodiment 210 of an ammonia recovery system is illustrated inFIG. 3 and may involve yet other integrations and combinations ofvarious components in order to simplify the overall system and otherwisestreamline operation. The third embodiment 210 may also be provided witha recycle tank 227 for preparing additional quantities of a make-upslurry composition 299.

Referring now to FIG. 3, with occasional reference to FIGS. 1 and 2 asrequired, third embodiment 210 may comprise a first or primary scrubber212 and a secondary scrubber 214. Primary and secondary scrubbers 212and 214 may be operatively associated with respective primary andsecondary slurry feed systems 216 and 218. The primary slurry feedsystem 216 may be operatively connected to a slurry atomizer 220provided within the primary scrubber 212. Secondary slurry feed system218 may be operatively connected to a slurry atomizer 222 providedwithin secondary scrubber 214. The primary and secondary slurry feedsystems 216 and 218 feed respective primary and secondary slurrycompositions 224 and 226 to the respective atomizers 220 and 222. Asalready described for the first and second embodiments 10 and 110, theprimary and secondary slurry compositions 224 and 226 may comprisemolybdic acid, but at different pH and specific gravity levels.

The third embodiment 210 may be used to remove ammonia contained in agas stream 230 produced by a chemical processing plant. Gas stream 230may result from the combination of a calciner vent stream and a tankvent stream, neither of which is specifically illustrated in FIG. 3, butwhich may involve the same arrangement as that illustrated in FIG. 1. Aswas the case for the FIG. 1 embodiment, the gas stream 230 of the thirdembodiment 210 will generally comprise a mixture of air, water vapor,ammonia, and carbon dioxide at compositions and temperatures alreadydescribed for the first two embodiments 10 and 110. After beingcollected and combined, the gas stream 230 is fed into the gas inlet 232of primary scrubber 212.

The primary scrubber 212 may be substantially identical to primaryscrubber 112 for the second embodiment 110 and may comprise a generallyvertically oriented, elongated chamber having a gas inlet 232 and a gasoutlet 238. Gas inlet 232 may comprise a generally elongated, tube-likestructure in which the atomizer 220 is positioned at about a distal end221 of inlet 232, as best seen in FIG. 3. This arrangement provides forenhanced and more efficient mixing of the inlet gas stream 230 andprimary slurry composition 224. Primary scrubber 212 may also comprise aprimary slurry tank 270 defined by the bottom portion of primaryscrubber 212.

Primary slurry tank 270 contains the primary slurry composition 224 andalso receives and collects further concentrated primary ammoniatedslurry 236 formed in primary scrubber 212. Primary slurry tank 270 mayalso be provided with an agitator 272 to prevent settling and ensure athorough dispersal of the solids in the primary slurry composition 224.Primary scrubber 212 may also be provided with a demister 241 to removequantities of liquid or mist contained in the partially processed gasstream 234 before it exits primary scrubber 212 via gas outlet 238. Inone embodiment, primary scrubber 212 may also be provided with acleaning system 243 which may be operated from time-to-time to spraywater 229 on demister 241 to prevent clogging thereof.

Secondary scrubber 214 may also comprise a generally verticallyoriented, elongated chamber having a gas inlet 240 and a gas outlet 246.The gas inlet 240 of secondary scrubber 214 may be connected to the gasoutlet 238 of primary scrubber 212 so that the partially processed gasstream 234 from the primary scrubber 212 may be fed into the secondaryscrubber 214. Gas inlet 240 of secondary scrubber 214 may comprise agenerally elongated, tube-like structure in which the atomizer 222 ispositioned at about a distal end 251 of inlet 240. This arrangementprovides for enhanced and more efficient mixing of the partiallyprocessed gas stream 234 and secondary slurry composition 226. The gasoutlet 246 from the secondary scrubber 214 may be vented to theatmosphere via a suitable blower assembly 264 and vent stack 266.

The secondary scrubber 214 may also comprise a combined or integratedarrangement in which the sump and secondary slurry tank are combined toform a single secondary tank 296 defined by the bottom portion ofsecondary scrubber 214. The combined secondary tank 296 holds thesecondary slurry composition 226 and also collects further concentratedsecondary ammoniated slurry 244 formed in secondary scrubber 214.Secondary tank 296 may be provided with an agitator 298 to preventsettling and ensure a thorough dispersal of the solids in the secondaryslurry composition 226. Secondary scrubber 214 may also be provided withtwo demisters 245 to remove quantities of liquid or mist contained inthe fully processed gas stream 242 before it exits secondary scrubber214 via gas outlet 246. Secondary scrubber 214 may also be provided witha cleaning system 247 which may be operated from time-to-time to spraywater 229 on demisters 245 to prevent clogging thereof.

As briefly mentioned above, the primary and secondary scrubbers 212 and214 may be operatively connected to respective primary and secondaryslurry feed systems 216 and 218. Primary slurry feed system 216 providesprimary slurry composition 224 to the slurry atomizer 220 of the primaryscrubber 212. The secondary slurry feed system 218 provides a secondaryslurry composition 226 to the slurry atomizer 222 of the secondaryscrubber 214.

Primary slurry feed system 216 may comprise a pump system 274 connectedbetween the primary tank 270 and the slurry atomizer 220 in the primaryscrubber 212. Pump system 274 may be used to deliver the primary slurrycomposition 224 to the slurry atomizer 220. A bleed outlet 248 allowsquantities of the primary slurry composition 224 to be withdrawn fromthe primary slurry feed system 216. The withdrawn slurry composition 224may be directed to an ammonia recovery system 252 for subsequenthandling and ammonia removal.

As was the case for the first two embodiments 10 and 110, primary tank270 may also be provided with a level control system 276 to maintain theprimary slurry composition 224 in primary tank 270 at the desired level.In the embodiment illustrated in FIG. 3, the level control system 276maintains the level of the primary slurry composition 224 in primarytank 270 by operating valves 286 and 285, i.e., to bleed primary slurrycomposition 224 from primary tank 270 or add secondary slurrycomposition 226 to primary tank 270 as necessary.

The primary slurry feed system 216 may also be provided with a pHcontrol system 282 for controlling the pH of the primary slurrycomposition 224. The pH control system 282 may be substantiallyidentical to the pH control systems 82, 182 of the first and secondembodiments 10 and 110. In the FIG. 3 embodiment, the pH control system282 maintains the pH of the primary slurry composition 224 by operatingvalve 286 to bleed primary slurry composition 224 from primary tank 270.

Primary slurry feed system 216 may also be provided with a densitycontrol system 237 to control the density of the primary slurrycomposition 224 contained within primary tank 270. Density controlsystem 237 may comprise a density meter, such as a Coriolis densitymeter (not shown). Density control system 237 also may be operativelyconnected to valve 283. Density control system 237 operates valve 283 tocontrol the flow of aqueous ammonia 281 into primary tank 270.

The primary slurry feed system 216 may be connected to a reagent tank223 via a pump 225. Reagent tank 223 supplies quantities of a make-upslurry composition 299 to primary and secondary tanks 270 and 296. Inone embodiment, reagent tank 223 may be configured to receive a supplyof make-up slurry composition 299 from a recycle tank 227. Reagent tank223 also may be configured to receive water (e.g., deionized water) 229,which may be used to control the specific gravity and/or pH of themake-up slurry composition 299. Reagent tank 223 also may be providedwith an agitator system 231 to ensure a thorough dispersal of the solidsin the make-up slurry composition 299.

Recycle tank 227 may be used to prepare quantities of make-up slurrycomposition 299 that may be fed to reagent tank 223 via pump 239. In theparticular embodiment shown in FIG. 3, recycle tank 227 may be connectedto water supply 229, a dry feedstock elevator 271, and a supply of aquaammonia (i.e., ammonium hydroxide) slurry 283. A suitable dry feed fromfeedstock elevator 271 may comprise molybdenum oxide. Recycle tank 227may also be provided with an agitator or mixer 273 to assist in mixingthe various constituents to form the make-up slurry composition 299.

The secondary slurry feed system 218 may be similar to the primaryslurry feed system 216 and may comprise a pump 211 connected between thesecondary tank 296 and the slurry atomizer 222. Pump 211 delivers thesecondary slurry composition 226 to the secondary scrubber 214 whereuponit is atomized by the slurry atomizer 222. The secondary slurry feedsystem 218 may be provided with make-up outlet 256 which allowsquantities of the secondary slurry composition 226 to be withdrawn fromthe secondary slurry feed system 218 and fed to the primary slurry feedsystem 216 via primary tank 270.

Secondary slurry feed system 218 may also be provided with a levelcontrol system 213 to maintain the secondary slurry composition 226 atthe desired level within secondary tank 296. In the third embodiment210, the level control system 213 maintains the level of the secondaryslurry composition 226 in secondary tank 296 by operating valves 285 and293, i.e., to bleed secondary slurry composition 226 from secondary tank296 or add make-up slurry 299 from reagent tank 223 as necessary.

The secondary slurry feed system 218 may also be provided with a pHcontrol system 217 for controlling the pH of the secondary slurrycomposition 226. The pH control system 217 may be substantiallyidentical to the pH control systems 17 and 117 of the first and secondembodiments 10 and 110. In the embodiment illustrated in FIG. 3, the pHcontrol system 217 maintains the pH of the secondary slurry composition226 by operating valve 285 to bleed secondary slurry composition 226from secondary tank 296.

The density of the secondary slurry composition 226 may be controlledvia a density control system 242. Density control system 242 maycomprise a density meter, such as a Coriolis density meter (not shown)and may be operatively connected to valve 275. Density control system242 operates valve 275 to control the flow of water 229 into secondarytank 296.

As mentioned above, quantities of the primary slurry composition 224that are bled from the primary slurry feed system 216 (i.e., via bleedoutlet 248) may be directed to an ammonia recovery system 252 as a feedliquor 254. As was the case for the first two embodiments 10 and 110,the ammonia recovery system 252 of the third embodiment 210 may compriseone or more filters and/or particle separation systems for removing (byfiltering) any impurities that may be contained in feed liquor 254.

Having herein set forth preferred embodiments of the present invention,it is anticipated that suitable modifications can be made thereto whichwill nonetheless remain within the scope of the invention. The inventionshall therefore only be construed in accordance with the followingclaims:

1. Apparatus for removing ammonia from a gas stream, comprising: aprimary scrubber having a gas inlet, a gas outlet, and a slurryatomizer, the gas inlet of said primary scrubber being operativelyconnected to the gas stream having ammonia contained therein; a primaryslurry feed system operatively connected to the slurry atomizer of saidprimary scrubber, said primary slurry feed system feeding to the slurryatomizer a primary slurry composition comprising molybdic acid at a pHlevel; a secondary scrubber having a gas inlet, a gas outlet, and aslurry atomizer, the gas inlet of said secondary scrubber beingoperatively connected to the gas outlet of said primary scrubber; asecondary slurry feed system operatively connected to the slurryatomizer of the secondary scrubber, said secondary slurry feed systemfeeding to the slurry atomizer a secondary slurry composition comprisingmolybdic acid at pH level, the pH level of the secondary slurrycomposition being lower than the pH level of the primary slurrycomposition, said apparatus atomizing the primary slurry composition inthe primary scrubber, the atomized primary slurry composition absorbingquantities of ammonia contained in the gas stream to produce a partiallyprocessed gas stream, the partially processed gas stream exiting saidprimary scrubber via the gas outlet and entering said secondary scrubbervia the gas inlet, said apparatus atomizing the secondary slurrycomposition in the secondary scrubber, the atomized secondary slurryabsorbing additional quantities of ammonia contained in the partiallyprocessed gas stream to produce a processed gas stream, the processedgas stream exiting said secondary scrubber via the gas outlet.
 2. Theapparatus of claim 1, wherein said primary slurry feed system comprisesa bleed outlet, said bleed outlet allowing quantities of the primaryslurry composition to be withdrawn from said primary slurry feed system.3. The apparatus of claim 2, wherein said secondary slurry feed systemcomprises a make-up outlet operatively connected to said primary slurryfeed system, said make-up outlet allowing quantities of the secondaryslurry composition to be withdrawn from said secondary slurry feedsystem and added to said primary slurry feed system.
 4. The apparatus ofclaim 1, further comprising: a primary pH control system operativelyassociated with said primary slurry feed system, said primary pH controlsystem controlling the pH level of said primary slurry composition; anda secondary pH control system operatively associated with said secondaryslurry feed system, said secondary pH control system controlling the pHlevel of said secondary slurry composition.
 5. The apparatus of claim 4,wherein said primary pH control system maintains the primary slurrycomposition at a pH level of at least about 7 and wherein said secondarypH control system maintains secondary slurry composition at a pH levelof at least about
 6. 6. The apparatus of claim 1, further comprising: aprimary specific gravity control system operatively associated with saidprimary slurry feed system, said primary specific gravity control systemcontrolling a specific gravity of said primary slurry composition; and asecondary specific gravity control system operatively associated withsaid secondary slurry feed system, said secondary specific gravitycontrol system controlling a specific gravity of said secondary slurrycomposition.
 7. The apparatus of claim 6, wherein said primary specificgravity control system maintains the primary slurry composition at aspecific gravity of about 1.3 or lower and wherein said secondaryspecific gravity control system maintains the secondary slurrycomposition at a specific gravity of about 1.4 or lower.